This invention generally relates to weld-bonding and more particularly to an improved weld-bonding of high surface resistance conductors separated by theremo-plastic insulation materials.
There are various and well known state of the art apparatus and methods for the weld-bonding of various structures. Each of these have achieved a degree of success.
S. Schnetzer, in U.S. Pat. No. 1,933,936 teaches a method of weld-bonding where penetration of the insulation material is achieved by gradually increasing the force between the electrodes without the use of any heat. This type of force penetration is undesirable as deformation of the materials by force results in stretch of the materials and resulting fractures.
P. Procacino, in U.S. Pat. No. 3,538,293 teaches reliance upon the electrical engagement between the electrodes and the conductors themselves to test for penetration and to apply weld current. This is found to result in a poor weld because of a wide range of material heat variance at the point of welding resulting in range of inconsistant bonds due to various different thickness and types of material. Procacino additionally teaches that a heat only stage of a fixed duration should be used while maintaining a close control over electrode temperature by using an external thermocouple controlled heating means embedded in the electrodes. This method requires special and expensive electrodes and it has been found that a fixed duration of melt heat can either be excessive or insufficient for a proper level of material heating depending upon the character of the material to be bonded and the type of adhesive used.
U.S. Pat. Nos. 3,317,407, 3,337,711, 3,614,375, 3,798,403 and 3,643,057 teach various other methods for performing weld bonding that include penetrating the thermo-plastic insulation material in the same or similar manner as discussed above.
After the thermo-plastic insulation material has been penetrated, state of the art resistance welding apparatus controls elevated the voltage across the electrode tips. The elevated voltage applied through the low resistance material results in an elevated weld current flow. After this current has flowed for a pre-selected time, it is terminated and an increased squeeze or forge pressure is applied to the electrode tips. It has been found that poor welds occur using this sequence of operation in that the material has not necessarily reached optimum temperature immediately after penetration and that in some instances the increase in material heat when weld current is applied causes an expansion greater then the first level of squeeze pressure resulting in expulsion of the weld nugget from the point of weld.
It would obviously be advantageous to improve weld-bonding by insuring consistant perfect penetration and weld by a simple and inexpensive improvement to now existing state of the art resistance welding apparatus.
Until the emergence of the instant invention, this could not be readily achieved.